The typical method of manufacture of cadmium electrodes for rechargeable cells is by impregnation of a sintered nickel plaque with an aqueous cadmium salt solution. This solution may contain various additives to facilitate the introduction of the cadmium salts into the plaque. Other components necessary for the later conversion of the cadmium salts to cadmium hydroxide in accordance with the particular process used are also included. The sintered nickel plaque does not constitute an active electrode, but merely provides a current carrier and support matrix for the active material, cadmium hydroxide.
Alternately, cadmium hydroxide may be applied as a paste to a current-carrying substrate of suitable material. Such a paste may begin as cadmium hydroxide or as cadmium oxide which later gains water to become cadmium hydroxide. It is customary in such paste systems to include metallic cadmium and/or nickel particles in order to provide a conductive matrix as disclosed in U.S. Pat. No. 3,775,273. While analogies with wet primary cells are frequently drawn, it should be noted that there are several major differences, including the fact that the cadmium electrode must function in a different electrochemical manner during the charge and discharge portions of its cycle. For such rechargeable cells, the cadmium hydroxide itself constitutes one electrode, the free hydroxyl ions form the electrolyte and active nickel hydroxide forms the second electrode. The physical substrates in such cells are merely current conductors to establish external contacts for the active electrochemical materials.
Subsequent to the manufacture of impregnated or pasted cadmium hydroxide electrodes it is normally necessary to execute a repeated charge discharge or formation cycle in a suitable electrolyte, such as aqueous sodium hydroxide solution. The function of this formation cycle is to produce cadmium metal particles or to convert those already included in the electrode to an electrochemically active form which provides a reserve of undischarged material. This formation cycle is carried out prior to assembly of the negative plates into rechargeable cells or prior to closing of the cells when carried out in situ with a large excess of electrolyte.
It has been recognized as desirable to be able to assemble a cell in its final usable state without need of this formation cycle, thus allowing construction of a cell from uncharged positive and negative electrodes. In general, two parameters determine the efficiency of cadmium metal incorporated in a battery electrode for the purpose of providing electrochemical precharge as a means of eliminating the formation cycle. These are: (1) Total surface area, and (2) particle size. In order to have electrochemical activity, a large surface is desired to provide sites for electrochemical reactions. In the case of metallic cadmium, this surface undergoes the discharge half-reaction: EQU Cd+20H.sup.- .revreaction.Cd(OH).sub.2 +2e.sup.-
during normal battery use.
It is apparent from this reaction that the cadmium metal must be in contact with a sufficient number of hydroxyl ions. Thus, the total surface area is critical for the precharge. But in addition, the battery must function over some period of time which may include several recharge cycles. In order to establish sufficient activity within a cell over the period of its life, there must be a sufficient cadmium reserve to make up for the generation of occluded pockets of cadmium formed during the charge cycle, which results in a condition known as "densification." By providing an electrochemical reserve of readily oxidizable cadmium material, a cell made from such a negative electrode matched with a suitable positive electrode will exhibit much less reduction in its capacity with successive charge-discharge cycles. This loss of the capacity is typical of cells which are lacking in readily oxidizable cadmium mass. It is believed that this phenomenon is caused by some of the cadmium material, produced from charging the active cadmium hydroxide, becoming occluded upon subsequent discharge, thereby resulting in inactivity. To maintain capacity through many cycles, a reserve from which active cadmium material may be drawn is provided.
In the past, cadmium metal particles have been produced in various shapes by differing methods. One method produces a finely divided "sponge-like" cadmium metal through electrolysis in conjunction with cadmium hydroxide which will, upon drying, produce a suitable mixture for manufacture of battery electrodes. However, there is no indication that the cadmium so produced has any significant electrochemical activity, and thus may not serve as an adequate precharge. The primary reasons for its use are to increase the bulk density of the electrode starting material and to provide an improved separation of the cadmium hydroxide particles so that the mixture is no longer sticky and may be easily poured.
Another type of cadmium particles is described generally as cylindroidal, but are more specifically dendritic, or tree-like, in structure as disclosed in U.S. Pat. Nos. 2,820,077 and 3,847,784. Acicular is defined as sharp, slender or needle shaped. It is known that the prior mentioned spongy cadmium made by electrolysis is unsatisfactory because it does not possess the necessary electrochemical activity for use in battery electrodes. The dendritic form of cadmium is made by the action of powdered aluminum or zinc on solutions of cadmium salts. This action produces the dentritic crystalline structures of the cadmium. While this form of cadmium may be more electrochemically active than cadmium from other processes, the dendritic structure is considered undesirable in batteries due to its marked propensity for causing short circuits to the positive plate as a result of additional crystalline growth during the battery cycle life.
Still another method of making cadmium particles is in the formation of non-spherical, irregular, gnarled, cylindroidal or sausage-like particles as described in U.S. Pat. No. 4,414,303.
One danger that is inherent in the use of small active particles is their pyrophoricity. The pyrophoricity or flammability of the powderized form of a given metal or material will vary with the process used to produce it. This is due to the surface area per unit weight ratio known as specific surface area of the material or the chemical reactivity of the material itself either of which may vary. High specific surface area makes a material more readily pyrophoric.
The desired cadmium metal structure for electrodes will have a high total surface for electrochemical activity, but a low specific surface area to reduce the pyrophoricity. It is known as disclosed in U.S. Pat. No. 3,870,562 that precharge activity can be produced by spherical particles in the specific size range of 3 to 12 microns made by a process of condensation from metallic cadmium vapors. These were considered to be small enough (large total surface area) to be electrochemically active, but large enough (lower specific surface area) not to be pyrophoric. However, it was found that beyond 12 microns, the cadmium particles have markedly diminished electrochemical activity and are not usable.
Furthermore, it is well known that the ignition temperature of a given particle size can be raised by a slight amount of oxidation; with the effect being most marked for the finer powders. Thus, some oxidiation can reduce the tendency for pyrophoricity. While the prior spherical powder is purported to be relatively safe, it is still necessary to take many precautions. What is missing is a pyrophorically safe, yet electrochemically active cadmium material for use as a negative electrode.